The goal is to understand the equal distribution of chromosomes to the daughter cells in mitosis and meiosis. An explanation for the normal success in chromosome distribution would also help explain the failures that result in Down's syndrome and other chromosome diseases in humans. Equal distribution depends upon directed chromosome movement in mitosis. The mitotic forces that produce directed movement would be measured in absolute units in living cells. The aims are to learn what causes chromosomes to move to the equator in prometaphase, where the mitotic motors are located, and to compare force production in prometaphase and anaphase. Microtubules in vitro and in a cell model will be micromanipulated to determine whether mitotic forces can affect microtubule structure and whether microtubule assembly generates force. Correlated studies of the same chromosome in living cells and in the electron microscope are proposed. The aims are to learn how the microtubule arrangement that produces equal chromosome distribution comes about and how tension forces stabilize the one appropriate arrangement of microtubules.